1. Field of the Invention
The present invention relates to a heat-shrinkable polyester film, and in particular to a heat-shrinkable polyester film that provides a shrink-wrapping display label for containers such as beverage or other bottles (i.e., tubular label, the same applies hereinafter) having an excellent heat shrinkage property and an excellent light blocking property even when the label is not auxiliary processed by printing.
2. Description of the Related Art
Recently, the use of shrinkable labels for the purpose of protecting the contents in bottles from ultraviolet ray has been expanding. Hitherto, polyvinyl chloride shrinkable films capable of blocking UV light have been commonly used for the purpose, but there exists an increasing need for a new heat shrinkable film made of other materials that blocks UV light. Specific light blocking properties required for the film may vary depending on the content in the bottle the film covers, but blockage of UV light in a longer wavelength range of 360 to 400 nm, especially of 380 to 400 nm is regarded as more important for protection of the content such as beverage and food, since the UV ray in the range brings about deterioration and discoloration thereof.
As such labels, heat-shrinkable films made of polyvinyl chloride, polystyrene, etc., have been commonly used (Japanese Unexamined Patent Publication No. 11-188817, etc.), but due to the problems such as emission of harmful chlorinated gases during combustion of wastes with regard to polyvinyl chloride, and a low printability of the labels with regard to polystyrene films, heat-shrinkable polyester films have recently attracting more attention as a new material for the purpose. However, there has been no practical film among the many conventional heat-shrinkable polyester films that could effectively block UV light in the longer wavelength range described above.
Additionally, colored PET (polyethylene terephthalate) bottles containing dyes and pigments have occasionally been used for protection of the contents in bottles. But these colored bottles are not suited for recycling, because the entire resins collected in the recycling process become contaminated with dyes and pigments contained in the colored bottles, and thus an alternative method has been searched for. To produce the same advantageous effect as that of using colored bottles, a combined use of a colorless bottle and a colored heat-shrinkable label covering the same bottle has been investigated as a possible alternative method.
When a heat-shrinkable film is used as a colored label for covering bottles, displays including words and designs are usually printed on an inner surface of the label, and the surface is then further covered with white paint with the intention of providing a UV light blocking layer, but-the thickness of the white ink-painted layer is typically about 3 μm, and not sufficient for that purpose. Although an attempt to print the entire surface of the film twice with white ink was made, such dual printed films too had drawbacks in quality (such as a change in the heat shrinkage property due to a larger thickness of the painted layer), leading to complexity in the label converting process, and thus elongation of the processing period and delay in delivery time.